Cloned forum/database changing bburl of one affects both - what's going on?

[SToP_GAP]

Hi, I'd appreciate any help anyone can offer here...

I have duplicated my forum folder and created a copy of the database with a different name. I've also created a new user in the database users with no access given to the original database. I have modified the config.php accordingly also.

The point of this exercise was to create a development clone of my live forum, to test a patch upgrade on it rather than the live site... nothing nefarious

I can only access one site at a time. If I change the bburl via the admin->options to the new forum then the new clone works fine, but, the original then ceases to work (I get a white page with no content). The admin panels remain functional at this point so if I now change the URL back on the original site, it then works fine but the clone is once again disabled.

Any changes to one seem to affect the other, despite the different databases and the user account for the clone being denied access to the original database. I'm a bit stumped here... can anyone offer any advice?

I should add that the content displayed on both sites is coming from the correct databases as posts are showing up on the original and not the clone.

I have tried clearing the cache table, no joy. xcache is installed but disabled right now. I've restarted lighttpd but nothing changed. The style being displayed is even different (as it should be, it was changed on the original after the data was exported and prior to importing it into the new clone database).